The use of portable electronic equipment, particularly mobile communication equipment has become widespread, resulting in the wish to supply new services employing different radio interfaces and/or protocols. This can be done by putting the functionality into modules such as SIM cards, which are regularly inserted in an apparatus such as a mobile telephone. Additional radio interfaces can thereby be established that fulfil, for example, IEEE WLAN 802.11, IEEE 802.15 (Bluetooth/Zigbee), IEEE 802.16 (WiMax) or similar standards, in addition to the normal functionality of the apparatus. Standards for so-called ISM (Industrial Scientific Medical) frequency band are of special interest, since by using these, extra services can be established on frequency bands where reasonably priced communication equipment is available. This type of integration of new functionality may also be envisaged in existing equipment by means of proprietary frequencies and protocols.
The invention presents a technical solution to the particularly challenging problem of implementing an electromagnetic coupler or antenna that has to be inserted in a unit of small physical dimensions. This was previously solved by inserting reactance elements such as inductors and coils in order to bring the small structure into resonance, but this kind of solution makes the antenna sensitive to the external environment, and any dielectric material or metal structures near the antenna will then influence the resonance frequency, thus making it impossible to obtain an effective radiation from the antenna. In such applications large metallic structures are located close to the antenna—often only 0.1-0.5 mm from the antenna elements. In addition the surrounding metal structure is of arbitrary geometry or shape, since the circuit board has to be arranged so as to be able to be inserted in various types of apparatus—also with a view to future, as yet unknown models of, for example, mobile telephones.
It is therefore a basic premise for the invention that under such conditions it is inappropriate to implement a conventional antenna solution.